"Error Compass Variance" & rover does not track straight

I am having two key issues with my rover. The build is a wild thumper 6WD frame, with a sabertooth dual motor controller, pixhawk (v2.5) and Taranis Plus radio and X8R receiver (SBUS to RCIN). I have the sabertooth set to independent mode (off,on,off,off,on,on) and SKID_STEER_OUT = 1. Whenever I am in manual mode or auto the rover does not want to track straight. It turns slightly to the right. I also have an “Error Compass Variance” error. I have included pictures of my build and some mission planner screen shots.

Thank you for the help.

I might have figured out the tracking issue. One of the motors is not spinning.
Issue I forgot to add. Whenever in Auto mode, mission planner thinks the rear is the front.

With regard to the rear being the front - when the rover is in motion does it appear to drive forward or backward on the MP screen? Which direction does it move in AUTO mode? Is the pixhawk and the compass module oriented the same way and are both facing forward - it looks like they are from the picture? It would be strange if it was backward in MP but the rover still navigated correctly in auto modes.
Thanks, Grant.